The present invention relates to digital signal processing, and more particularly to the digital prefiltering of encoded video signals to minimize cross color and dot crawl errors when the encoded video signals are subsequently decoded.
An encoded video signal has a chrominance component, formed by modulating quadrature components of a subcarrier frequency, and a luminance component that is added to the chrominance component. Two major types of errors occur when such an encoded video signal is decoded: cross color errors where luminance energy is added to the decoded chrominance component, and dot crawl errors where chrominance energy is added to the decoded luminance component. Diagonal luminance patterns at or near the color subcarrier frequency are impossible to properly decode, with some or all of the energy being mistaken for chrominance information regardless of the type of decoder being used. Dot crawl errors caused by chrominance transitions are somewhat easier to minimize in the decoder, but in practice they cannot be entirely eliminated. Large magnitude chrominance changes with fast risetimes produce the most dot crawl errors when the signal is decoded. Simple single (1H) or double (2H) horizontal line comb decoders do reduce cross color errors while preserving luminance detail, but cross color errors are still often visible. The dot crawl errors on horizontal lines and edges are a side effect of the comb filtering. Such introduced dot crawl errors are reduced by placing a subcarrier notch filter in the path of the decoded luminance component, but this softens detail. What is desired is a method of encoding the luminance and chrominance components within an encoder to reduce errors when the encoded video signal is decoded without any unnecessary loss of detail in the image represented by the encoded video signal.